1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic recording technique, such as a copying machine or a printer, and, in particular, to an image forming apparatus that performs image formation using at least two kinds of toner of substantially the same hue and different coloring agent contents.
2. Description of the Related Art
In recent years"" electrophotographic image forming apparatuses, of which an image quality akin to that of silver salt photography is demanded, an improvement in terms of resolution and gradation is an issue that has become more important than ever.
Examples of a method of obtaining an image of high gradation include a dither method, a density pattern method, and a PWM method. Solid image areas, halftone image areas, and line areas are expressed by varying dot density, respectively.
However, it is difficult to place toner particles with high fidelity on dots formed by a laser beam corresponding to image information, and the toner particles may be deviated from the dots. Thus, such a problem is liable to occur that it is impossible to obtain, regarding a toner image, gradation reproducibility in correspondence with a dot density ratio of black and white areas of a digital latent image.
Further, in the case in which, to achieve an improvement in terms of image quality, an attempt is made to achieve an improvement in terms of resolution by diminishing a dot size, reproducibility for the latent image, formed of minute dots, suffers, making it rather difficult to stabilize the gradation reproducibility for a highlight image area.
Further, the irregular disturbance in the dots is perceived as granularity, which leads to deterioration in the image quality of the highlight image area.
To solve the above problems, there has been proposed a method, in which the highlight image area is formed by using light color toner, and in which the solid image area is formed by using deep color toner. For example, JP 11-84764 A and JP 2000-305339 A disclose an image forming method according to which image formation is effected by using a combination of a plurality of toners of different densties.
Further, JP2000-347476A discloses an image forming apparatus in which deep color toner is combined with light color toner whose maximum reflection density is not more than half the maximum reflection density of the deep color toner. JP 2000-231279 A discloses an image forming apparatus in which deep color toner exhibiting an image density of 1.0 or more is combined with light color toner exhibiting an image density of less than 1.0 when the amount of toner on the transfer material is 0.5 mg/cm2. JP 2001-290319 A discloses an image forming apparatus in which deep color toner and light color toner whose recording density inclination ratio ranges from 0.2 to 0.5 are combined.
By thus developing the highlight image area by using light color toner, it is possible to achieve an improvement in terms of image quality of the highlight image area, which has been a problem in a high resolution digital full-color electrophotographic apparatus.
It is to be noted, however, that in the halftone image area, in which a deep-colored toner image is slightly superimposed on a light-colored toner image, when the dots formed by the deep color toner are large, the dots of the deep color toner become conspicuous, resulting in a deterioration in granularity. Further, owing to this deterioration in granularity, it is impossible to maintain smooth gradation corresponding to the image information, resulting in appearance of a noise such as a false contour.
The present invention has been made in view of the above problems in the prior art. It is an object of the present invention to provide an image forming apparatus capable of eliminating granularity from an output image.
Another object of the present invention is to provide an image forming apparatus superior in gradation characteristics.
Still another object of the present invention is to provide an image forming apparatus including:
a first light source emitting a beam corresponding to image information;
a first photosensitive means receiving the beam emitted from the first light source;
a first developing means for developing a latent image formed on the first photosensitive member with a first toner;
a second light source emitting a beam corresponding to image information;
a second photosensitive member receiving the beam emitted from the second light source; and
a second developing means for developing a latent image formed on the second photosensitive member with a second toner,
in which a coloring agent contained in the first toner and a coloring agent contained in the second toner are substantially of the same hue, with the content of the coloring agent contained in the second toner being smaller than the content of the coloring agent contained in the first toner, and
in which an oscillation wavelength of at least the first light source ranges from 370 to 500 nm.
Further objects of the present invention will become apparent from the following detailed description given with reference to the accompanying drawings.